The funnyman, who portrays the family patriarch on beloved sitcom Full House, is currently on a stand-up comedy tour across America, and happened to have a stop in Milwaukee, Wisconsin over the weekend (ends13Mar16) - just minutes away from where Carly Rae had her own gig in the city.
The singer, who re-recorded the show's classic theme Everywhere You Look for the Full House spin-off, was unaware Bob was also in town until he unexpectedly took the stage, much to the screaming approval of the equally surprised crowd.
Carly Rae posted a photo on Instagram of Bob down on one knee as if he was popping the question to her on stage, and she added the caption, "Tonight @bobsaget proposed and I said YES! Jk (just kidding) but he did surprise me on stage and then of course we sang the FULL/FULLER house theme song together!"
She also reposted a snap Bob had shared on his own account, which he captioned: "Great night in Milwaukee hopping onstage after my super fun show @PabstTheater then ran to the Turner Hall Ballroom to hop onstage with the amazing @CarlyRaeJepsen and I attempted to sing with her the #FullerHouse theme, which she sings for the new @Netflix @FullerHouse."
He also shared another photo of himself with mouth agape as he belted out the words to the tune, writing, "You'd a thunk I was trying to sing an Aerosmith song, but alas, I was attempting to sing the #FullerHouse theme with the wonderfully talented lady who actually sings it on @FullerHouse (sic)."
Carly Rae, 30, recently gushed to Rolling Stone magazine about being chosen to re-record the iconic TV theme, divulging, "Full House is essentially my childhood. It was my favourite TV show growing up. So I was offered this and wondering if it could fit into my schedule; I had a lot going on at the time.
"I remember my team thinking, 'I don't think you're going to have time to do this.' It was casually mentioned in one of those phone calls, and I said, 'What? We have to make time for this! I'll come in the middle of the night, whatever it is.'"